1. Field of the Invention
The present invention relates to an image pickup optical system and, more particularly, to the image pickup optical system for silver-salt cameras and electronic image pickup cameras.
2. Related Background Art
Materials forming the optical system are affected by a change in the surrounding circumstances. When optical materials, particularly organic materials, are used for the optical system, it is generally known that they absorb water vapor in the atmosphere or discharge water out of the optical system, so as to change the refractive index and dimensions of lens, thereby varying the characteristics of the optical system.
A known example of countermeasures against it is, as disclosed in Japanese Patent Application Laid-Open No. 3-181908, a method capable of maintaining constant optical characteristics without being affected by a change of humidity in the atmosphere, by employing such structure that each element made of the organic material is placed in a frame and that the outermost frame in contact with the atmosphere is made of an inorganic material (which should be an optical glass from the description of the optical characteristics in the embodiment, though the specification of the application describes only that it is a non-hygroscopic material) and is covered by a lid so as to maintain the inside humidity at that upon assembly.
Further, Japanese Patent Application Laid-Open No. 7-128707 (corresponding to U.S. Pat. No. 5,581,400) discloses a method of using a low-hygroscopic material as a material for forming the optical system.
However, the method described in Japanese Patent Application Laid-Open No. 3-181908 necessitates the optical component used as a lid, which increases the number of components.
Japanese Patent Application Laid-Open No. 7-128707 describes only the finder optical system, but describes nothing about the image pickup optical system in particular.
It was described previously that absorption of moisture caused the change in the refractive index and dimensions of the optical material. Describing in further detail, the ordinary optical materials increase the refractive index and dimensions with moisture absorption. Since the water in the air is absorbed from the surface of the medium to diffuse into the inside of the medium, the moisture-absorbing phenomenon induces an index difference between the medium surface and the inside even in the medium having the uniform internal index. After that, the moisture absorption reaches a state of equilibrium, so that the index becomes uniform again. As for the dimensional change due to the moisture absorption, the dimensional change appears only in the surface of the medium in the initial stage of the moisture absorption, and thus, stress appears inside the medium to change the surface shape. When the moisture absorption reaches equilibrium, the entire medium expands uniformly and the deformation of the surface shape is settled, too. For these reasons, even an optical component originally having no refractive power will come to have a refractive power with the absorption of moisture and again turn into a non-optical power material as the moisture absorption approaches the equilibrium.
The following points can be listed as influencing the index distribution performance of optical components.
1. An index gradient in the direction of the optical axis changes only the optical path length but rarely affects the power. PA1 2. An index gradient in a direction normal to the optical axis greatly affects the power and gives rise to strong spherical aberration, particularly, in the initial stage of moisture absorption. PA1 3. An index gradient in a direction inclined with respect to the optical axis brings about a state between above 1 and 2 and the influence on the power thereof varies depending upon the direction. PA1 4. If there is a difference between index gradients in mutually orthogonal directions normal to the optical axis, powers differ depending upon azimuths and an astigmatic difference also appears on the optical axis. PA1 5. The influence becomes greater on the power as the optical path becomes longer in the medium. PA1 6. In the case of an optical element having a smaller size, the time to the equilibrium is shorter, but the index gradient becomes steeper in the moisture- absorbing process; therefore, a temporal change amount of power becomes greater. PA1 an optical element of a transparent, optical material comprising an entrance surface, at least one reflective surface, and an exit surface, the optical element having an optical power; and PA1 an image pickup element comprising a plurality of pixels, wherein on the image pickup element an image is formed by light from an object through the optical element, PA1 wherein the optical material is a material having an index change amount .DELTA.n from an absolute dry condition at the temperature of 50.degree. C. to saturation under a circumstance of the temperature 50.degree. C. and the humidity 90%, the index change amount .DELTA.n satisfying the following condition: EQU .DELTA.n.ltoreq.F.delta.r.sub.0.sup.2 /(2f.sup.2 L) PA1 an optical element of a transparent, optical material comprising an entrance surface, at least one reflective surface, and an exit surface, the optical element having an optical power; and PA1 an image pickup element comprising a plurality of pixels, wherein on the image pickup element an image is formed by light from an object through the optical element, PA1 wherein the optical material is a material having an index change amount .DELTA.n from an absolute dry condition at the temperature of 50.degree. C. to saturation under a circumstance of the temperature 50.degree. C. and the humidity 90%, the index change amount .DELTA.n satisfying the following condition: EQU .DELTA.n.ltoreq.F.delta.d.sup.2 /(2f.sup.2 L)
Incidentally, in the finder optical system of ordinary compact cameras, it is common practice to use an eyepiece having the focal length of about 18 mm. Supposing a permissible amount of power change due to the moisture absorption is one diopter, a permissible change of the focal length due to the moisture absorption is about 0.3 mm. On the other hand, for example, in image pickup optical systems using solid state image sensing devices, as typified by digital cameras, the image pickup devices have been decreasing their size but increasing the number of pixels in recent years and with such progress demands are increasing for further downsizing and higher accuracy of the image pickup optical systems. For example, in the case of the image pickup optical system having the diagonal pickup device size of a quarter inch, using an image pickup device of about 800,000 pixels (at the pixel pitch of about 5 .mu.m), and having the F-number of about 2, the depth of focus is approximately 10 .mu.m. It is then necessary to suppress the variation of the focal length due to the moisture absorption to below 10 .mu.m. Since the influence of moisture absorption on the power becomes more significant with a decrease in the size of optical element as described previously, the moisture absorption poses a significant problem in the image pickup systems decreasing their size.
On the other hand, it has been clarified that an optical system with sufficient correction for aberration can be constructed by introducing the concept of a reference axis and forming a component surface of an asymmetric, aspherical surface even in a non-coaxial, optical system; for example, Japanese Patent Application Laid-Open No. 9-5650 describes its designing method and Japanese Patent Applications Laid-Open No. 8-292371 and No. 8-292372 (both corresponding to U.S. Pat. No. 5,825,560) describe its design examples.
The moisture-absorbing phenomenon is a diffusion phenomenon of water particles into the inside of a medium and the index distribution inside varies depending upon the distance from the surface of the medium. It is thus expected in the case of a lens of a simple shape, for example, a lens of an almost spherical shape that equals index surfaces of refractive indices inside the lens also have the shape close to a sphere. In the optical system, for example, as described in Japanese Patent Application Laid-Open No. 8-292371, however, because it has a complicated structure in which a plurality of optical surfaces are formed integrally, the inside index distribution thereof is also complex. In the optical system described in this Japanese application the optical path is bent several times via reflective surfaces inside the optical element. Since the light passes twice in the index distribution before and after each reflection inside the medium, the influence of the index distribution on the optical performance is greater than in the case of only refractive surfaces being used. In the non-coaxial optical system, which is an optical system in which even a reference axis ray is obliquely incident to the optical surfaces as in the optical system described in the above Japanese application, the influence of index distribution is greater on the optical performance, because the light passes through different optical paths having different index profiles upon incidence to and upon emergence from the reflective surfaces. When the reflective surfaces have curvature as in the optical system described in the same Japanese application, the influence of moisture absorption on the optical performance becomes greater than in the cases using flat reflective surfaces, because the equal index surfaces inside the medium also have curvature. When the asymmetric, aspherical surfaces are used for the optical surfaces as in the optical system described in the same Japanese application, the inside index distribution is also asymmetric and thus asymmetric aberration occurs due to the moisture absorption.
In the optical system described in the embodiment of Japanese Patent Application Laid-Open No. 7-128707, all the reflective surfaces used are flat reflective surfaces and nothing is described as to the countermeasures against the issues in the use of the reflective surfaces having the curvature as described above.